We will systematically examine, in normal rats, presynaptic inhibitory and facilitory modulation of noradrenergic transmission in two selected blood vessels, the portal vein and caudal artery, and three areas of the central nervous system which are intimately involved in the regulation of blood pressure; a) anterior and posterior hypothalamus, b) the A1 and A2 areas of the brain stem and c) the spinal cord. In addition we will examine such modulation of adrenergic transmission in the A1 and A2 regions of he brain stem. This will be done by assessing the effect of inhibitory and facilitory agonists and antagonists on the electrically induced release of 3H-norepinephrine (NE) and 3H-epinephrine (Epi) from superfused tissue. We will further determine if there are qualitative or quantitative differences in the activity of these presynaptic receptor - during the development or maintenance of hypertension in the spontaneous hypertensive rat (SHR), DOCA-salt hypertensive rat, renal hypertensive rat or rats made hypertensive by lesions of the nucleus tractus solitarii. Qualitative or quantitative differeces in presynaptic receptors will also be assessed following changes in dietary salt intake. The role of presynaptic receptors and NE synthesis as well as the role of impulse flow on NE release in vascular smooth muscle will also be explored. It is thought that these studies will provide valuable information on the role of presynaptic receptors in the normal reulation of synthesis and release of these catecholamines in vascular smooth muscle and the CNS and whether or not alterations in the normal functions of presynaptic receptors contribute to the development or maintenance of some types of hypertension.